Human red blood cells will be studied in vitro in a series of experiments designed to help our understanding of the molecular mechanisms of transport. Three areas of study are proposed: 1) Lithium-sodium exchange will be measured under the conditions needed to distinguish sequential from ping-pong kinetic mechanisms. 2) The inhibition of chloride transport by preincubation with phospholipid vesicles will be measured under conditions necessary to determine the role of calcium entry into cells and phospholipid exchange in this inhibition. 3) The kinetics of the active calcium pump will be measured in inside-out vesicles from red blood cells under different conditions of pH, calcium concentration, inhibitors and the activator, calcium dependent regulatory protein.